Free Online HTTP Headers Reference Guide

Understanding HTTP Headers

HTTP headers are key-value pairs sent between clients and servers to provide essential information about the request or response. They enable content negotiation, caching, authentication, security policies, and much more. Understanding HTTP headers is fundamental to web development, API design, and troubleshooting network issues.

Headers are categorized into request headers (sent by the client), response headers (sent by the server), representation headers (describing the message body), and payload headers (containing information about the payload data). Modern web applications rely heavily on headers for implementing security measures, optimizing performance through caching, and enabling cross-origin resource sharing.

Request vs Response Headers

Request headers are sent by the client (browser, mobile app, or API client) to provide context about the request. Common examples include Authorization for authentication credentials, Accept to specify desired content types, User-Agent to identify the client software, and Cookie to send stored cookies back to the server.

Response headers are sent by the server to provide information about the response. These include Content-Type to specify the media type of the returned content, Set-Cookie to store cookies on the client, Location for redirects, and Server to identify the server software. Response headers also control caching behavior and define security policies.

Security Headers

Security headers are critical for protecting web applications from common attacks. Content-Security-Policy prevents XSS attacks by restricting resource loading. Strict-Transport-Security enforces HTTPS connections. X-Frame-Options prevents clickjacking by controlling iframe embedding. X-Content-Type-Options stops MIME-sniffing attacks. Modern applications should implement comprehensive security header policies to protect users and data.

Caching Headers

Caching headers control how and when content is cached by browsers and intermediate proxies. Cache-Control provides fine-grained control with directives like max-age, no-cache, and private. ETag and If-None-Match enable efficient conditional requests. Last-Modified and If-Modified-Since provide date-based validation. Proper caching reduces server load, improves performance, and enhances user experience.

CORS Headers

Cross-Origin Resource Sharing (CORS) headers enable secure cross-origin requests from web browsers. Access-Control-Allow-Origin specifies which origins can access the resource. Access-Control-Allow-Methods defines permitted HTTP methods. Access-Control-Allow-Headers lists allowed request headers. Access-Control-Allow-Credentials controls whether credentials can be included. CORS is essential for modern web applications that consume APIs from different domains.

This reference guide provides comprehensive information about 50+ HTTP headers used in modern web development. Each header includes a description, practical examples, common values, and links to official RFC specifications. Use the search and filter tools to quickly find the headers you need.